1. Field of the Invention
The present invention relates to an inkjet printhead which prints by discharging ink droplets, and a printing apparatus.
2. Description of the Related Art
In inkjet printing using an inkjet printhead (to be referred to as a printhead), the discharge amount during printing varies due to variations of the head temperature, and unevenness appears in the image density. As an arrangement for suppressing occurrence of the density unevenness, there is proposed a divided-pulse width modulation driving method of supplying a divided pulse as a drive pulse to be applied to a heating element (heater) (see Japanese Patent Laid-Open No. 8-156256).
According to this method, a preheat pulse having a variable pulse width is applied, and then a main heat pulse is applied to drive the heating element after the interval time. A preheat pulse P1 is mainly used to control the ink temperature within the nozzle, and its pulse width is changed depending on a temperature detected by the temperature sensor of the printhead, or the like. Also, the preheat pulse P1 is used for heating the heater to an extent that ink is not discharged. An interval time P2 is a predetermined time interval set to prevent the preheat pulse and main heat pulse from mutually interfering with each other. By setting the interval time P2, the temperature distribution of ink within the nozzle is made uniform. A main heat pulse P3 is used to generate bubbling by the heating element and discharge an ink droplet from the orifice. The divided-pulse width modulation driving method can considerably reduce density variations and density unevenness caused by variations of the discharge amount during printing.
However, ink is not always discharged using all nozzles during printing. For example, printing may be performed using nozzles on only one half side of the printhead or using only part of the printhead in accordance with image data. When nozzles within the printhead are used not uniformly but locally, even the temperature distribution within the printhead is also localized. As a result, partial printing density unevenness occurs.
To prevent this, there is a method in which a heating element for discharging an ink droplet is driven by a pulse short enough not to discharge an ink droplet so that the printhead temperature falls within a predetermined range before or during the print operation (short-pulse heating).
A drive pulse for discharging an ink droplet will be called a normal drive pulse, and a head temperature retaining pulse short enough not to discharge an ink droplet will be called a short pulse.
Japanese Patent Laid-Open No. 8-156256 discloses an arrangement in which, for a nozzle which receives a printing signal, at least one of P1, P2, and P3 defining a drive pulse is modulated based on temperature information of a temperature sensor or the like by a pulse generation circuit for generating the normal drive pulse, and is applied to the heating element. For a nozzle which does not receive a printing signal, a pulse short enough not to cause bubbling is generated as only the drive pulse P1 or P1 and P3, and applied to the heating element.
Japanese Patent Laid-Open No. 2003-89196 discloses an arrangement in which a pulse generation circuit for generating the short pulse is arranged in addition to a pulse generation circuit for generating the normal drive pulse. The pulse generation circuit for generating the short pulse in Japanese Patent Laid-Open No. 2003-89196 generates the short pulse by calculating the logical product of PWM set to the cycle Tc and duty Ton and an internal pulse having an assert time width Tpn corresponding to the short pulse generation count. By the same method as that in Japanese Patent Laid-Open No. 8-156256, a pulse is applied to a nozzle which receives a printing signal. For a nozzle not used in printing, successive short pulses are applied to the heating element by an arbitrary count not to cause bubbling, based on Tpn, Tc, and Ton set by the short pulse generation circuit.
At present, inkjet printing apparatuses (to be referred to as printing apparatuses) need to increase the numbers of ink colors and nozzles used in the printhead and the length of the printhead for higher printing speed and higher image quality. However, to satisfy these requirements, the number of control signals and the signal amount to be supplied to the printhead also increase. For a compact, high-speed apparatus, it is necessary to increase the signal transfer speed and decrease the number of signal lines.
In particular, a larger number of signal lines lead to a larger number of head contacts for finally connecting the head substrate. This inhibits reduction in the cost and profile of the printhead.
To decrease the number of head contacts, control circuits are arranged on respective substrates regarding head control, and head control signals are serialized and transmitted/received between the respective substrates, decreasing the number of signal lines which finally reach the head substrate. In parallel to this, the transfer speed is increased by data transfer using LVDS (Low Voltage Differential Signaling).
A control circuit on the head substrate has a function of deserializing serial data and even a function of controlling generation and supply of a drive pulse to be applied to the heating element of the head. A control circuit arranged on the control substrate of the printing apparatus main body outputs drive pulse control parameters to the control circuit on the head substrate together with a print data signal.
For example, the following parameters are necessary in the presence of two pulse generation circuits, that is, a pulse generation circuit for generating the normal drive pulse and a pulse generation circuit for generating the short pulse, as in Japanese Patent Laid-Open No. 2003-89196. More specifically, generation of the normal drive pulse requires, as control parameters, parameters capable of specifying the time widths of P1, P2, and P3. Generation of the short pulse requires, as control parameters, parameters capable of specifying the cycle Tc, duty Ton, and count Tpn. Since different control parameters are necessary, a signal line for input to the pulse generation circuit for generating the short pulse needs to be arranged in addition to a signal line for input to the pulse generation circuit for generating the normal drive pulse. However, this increases the number of head contacts.
In contrast, the number of control parameters for operating the pulse generation circuit is decreased by controlling the short pulse by the pulse generation circuit for generating the normal drive pulse, as in Japanese Patent Laid-Open No. 8-156256. This is advantageous in minimizing the number of signal lines and the data bit length. This method was needed and desirable at a time when the method disclosed in Japanese Patent Laid-Open No. 8-156256 was proposed. However, to cope with recent increasing numbers of nozzles mounted on the printhead and ink colors used, a demand has arisen for the pattern of a short pulse waveform with high degree of freedom in adjustment of the pulse generation count and the like.
To meet this demand, the pulse generation circuit for generating the normal drive pulse may be expanded so that it can output various waveforms. However, a set of control parameters necessary to set many pulse change points needs to be prepared by the number of change points. Thus, the bit length of input data which defines the control parameter set increases, resulting in a long data transfer time.